1. Field of the Invention
The present invention relates to an image pickup apparatus to which an exchangeable lens system is set and capable of picking up a still image and a moving image.
2. Related Background Art
A maximum power point tracking system is conventionally known as an automatic focus regulation system used for an image input unit such as a video camera.
This is a system for extracting a high frequency component in an image signal obtained from a solid-state image pickup device such as a CCD image pickup device, driving an image pickup lens so that the high frequency component is maximized and regulating a focal point.
This automatic focus regulation system does not require a special optical member for focus regulation and has an advantage of accurately brining into focus independently of a distance even for a distant place or near place.
A maxim-power-point-tracking-type automatic focus regulation system is briefly described below by referring to FIG. 19.
The light from an object enters the image pickup face (photoelectric conversion face) of an image pickup device 120 by passing through a fixed first lens group 110, second lens group 112 for variable power (hereafter referred to as variable power lens), aperture stop 114, fixed third lens group 116 and fourth lens group 118 (hereafter referred to as focusing lens) having a focus regulation function and a function for correcting movement of a focus face in accordance with variable power.
The image pickup device 120 converts an optical image on the image pickup face into an electrical signal.
An output signal for the image pickup device 120 is sample-held by a CDS circuit 122, amplified to a predetermined level by an AGC circuit 124 and converted into a digital signal by an A/D converter 126.
An output signal of the A/D converter 126 is supplied to a not-illustrated camera signal process circuit.
Because the process content of the camera signal process circuit is publicly known but it is not related with the present invention, further description thereof is omitted.
An output of the A/D converter 126 is also applied to a band-pass filter (BPF) 128.
The BPF 128 extracts a predetermined high-frequency component from the image data output from the A/D converter 126.
Every output of the BPF 128 is converted into a straight-polarity signal by an ABS circuit 130.
A gate signal generation circuit 132 generates a gate signal for designating a portion corresponding to the inside of a focus detection area in an image pickup screen.
A phase detector 134 phase-detects (for example, peak-holds or integrates) only a signal corresponding to the inside of the focus detection area from an output of the ABS circuit 130 in accordance with the gate signal.
The phase-detected signal is output at an interval synchronizing with integral multiples of a vertical sync signal as an AF (automatic focus regulation) evaluation value.
A main control circuit 136 constituted of a microcomputer captures an output (AF evaluation value) of the phase detector 134.
Focusing speed corresponding to a focusing degree and a motor driving direction according to an increment of AF evaluation value are decided based on the captured signal and a motor driving circuit 138 is controlled based on the decided result.
The motor driving circuit 138 drives a focus motor 140 in accordance with an instruction from the main control circuit 136 to move the focusing lens 118 to a designated position at a designated speed.
Thereby, the focusing lens 118 is controlled at a position where an output of the BPF 128 is maximized.
The main control circuit 136 also rotates a zoom motor 144 by a motor driving circuit 142 to move the variable power lens 112 up to a designated position in accordance with a variable power operation by a user.
Thereby, it is possible to change focal lengths and image pickup magnifications are changed.
FIG. 20 shows a flowchart of the maximum-power-point-tracking-type automatic focus regulation system by the main control circuit 136.
The main control circuit 136 continuously captures outputs (AF evaluation values) of the phase detector 134 at intervals synchronizing with integral multiples of a vertical sync signal to execute automatic focus regulation control.
When power supply is turned on or an image pickup preparation mode is started, AF return control is started (S1) and the focusing lens 118 is driven in a direction in which an AF evaluation value increases to perform the maximum power point tracking control (S2).
By overshooting and the returning the apex of a mountain (maximum point of power), the apex of the mountain is determined (S3) and stopped at a highest-level point to wait for restart (S4). When detecting that the level of the AF evaluation value lowers from the level at the time of stop, the AF return control is restarted (S5).
Moreover, in addition to a system for extracting a high-frequency component in an image signal by the BPF 128, a configuration of extracting a high-frequency component from a conversion result of a two-dimensional orthogonal converter used for image compression and using the component for automatic focus regulation is also proposed in recent years.
Furthermore, Japanese Patent Application Laid-Open No. H08-327893 discloses a moving focus regulator for excluding the influence of a spatial frequency of an object whose focus will be detected and always performing focus regulation in accordance with a best image face position without changing focus detection optical system.
The conventional maximum-power-point-tracking-type automatic focus regulation system is described above. In recent years, however, change in lenses to zoom and high magnification and change in image pickup means to high pixel and high density are accelerated for image pickup of a camera.
Moreover, an image pickup apparatus capable of picking up a moving image and a still image and selecting the number of recording pixels in still image pickup and a high-vision (HD) image pickup apparatus also in moving images are spread.
In the case of an exchangeable lens system, a camera and lens advance in improvement of the resolution of an image pickup apparatus and image quality with the times and evolve to various conformations.
For example, though an early exchangeable lens system corresponds to image recording of only a standard TV signal, a camera and lens capable of picking up a resolution still image higher than a TV signal and high vision are developed and put on sale and new and old camera lenses are present.
It is preferable that combination of them is performed without problem in image pickup in combination of marketability, merchandise property and compatibility.
For example, when a camera corresponds to HD and image pickup cannot be performed by even combination with an old lens designed for standard TV, the marketability and merchandise property as an exchangeable lens system are extremely deteriorated.
However, in the case of this combination, because a camera is high-resolution vision, a spatial frequency to be resolved as shown in FIG. 5 is high.
Therefore, a high extraction frequency characteristic of high frequency component to be detected from an image signal is also necessary compared to a conventional TV system.
In some cases, there may be a case in which proper AF cannot be performed unless there is further information. However, in the case of this example, the lens only has a resolution for standard TV and an AF system may be present in the lens.
However, the AF system only has a system for conventional standard TV.
In the case of focus detection means, it is necessary to extract the high-frequency component in the image signal as described above. However, the frequency to be extracted is usually previously decided by a camera.
For example, when a focus signal of a camera is set for high vision in accordance with the above combination, a case in which characteristic is not satisfied and the operation of AF may not be proper.
Moreover, when an AF system is present in a lens, it is possible to perform only AF for conventional TV.
Therefore, it is considered that the AF does not properly operate or does not operate at all.
That is, because a signal to be extracted has a high frequency, a signal change is small when the peak is greatly deviated compared with a case where a frequency to be extracted is low (when focus position is horizontally deviated from the peak position of a high-frequency output signal) as shown in FIG. 6 and response is delayed.
Moreover, a case may occur in which the peak of the fluctuation of the aberration of a lens is detected because the focusing operation is performed by a signal equal to or higher than the resolution of a lens and AF is stopped at a position of not focusing an object.
However, a case may occur in which a lens for high vision is used, a camera for standard TV is used, AF is a lens for high vision and signal for focusing is generated by the camera and transmitted to the lens.
In this case, because the lens receives only a signal for standard TV, when performing AF for high vision, a case may occur in which AF starts from a greatly deviated state or operation nearby focusing is not properly performed.
Moreover, a case is assumed in which a camera capable of picking up a still image and NTSC-type moving image is used, a lens has an AF function regulated to NTSC, the focusing signal of the camera is regulated to an NTSC-type frequency to be extracted and a still image is recorded at higher pixel and higher density than in the case of NTSC type.
FIG. 7 is an illustration showing comparison between spatial frequency characteristic to be resolved of NTSC and spatial frequency to be resolved when recording images at high pixel and high density when picking up a still image.
As shown in FIG. 7, it is assumed that the spatial frequency to be resolved in the case of NTSC is as NTSCHz and a still image is picked up at 3 million pixels more than a necessary spatial frequency of NTSC.
In this case, the spatial frequency to be resolved “Still-300” Hz becomes higher than the spatial frequency to be resolved for NTSC.
In this case, it is assumed that the spatial frequency to be resolved is not a frequency at a limit in which the frequency can be resolved but a frequency having high enough MTF is an object spatial frequency to be resolved as indicated by the arrows in FIG. 7.
Though this deciding method is optional, sufficient MTF is obtained when setting approx. 80% of a limit resolution spatial frequency as a target.
Therefore, when picking up a still image of 3 million pixels by using the spatial frequency of “NTSC” Hz to be resolved of NTSC as a frequency characteristic for focusing detecting of AF, defocusing can be recognized due to the difference of resolution limit of the spatial frequency.
That is, it is impossible to detect the peak of a focus for the object of “Still-300” Hz.
Also in this case, a problem same as a case of combination of standard TV and HD previously described occurs.
That is, it is difficult to detect a focus position from a defocusing state and as a result, response characteristic is deteriorated. Similarly, because positional width of peak is narrow nearby the peak, a problem in lacking in stability occurs by overshooting a peak position.
However, by taking only still image pickup as an example, in the case of a camera having a plurality of settings of a picked-up image such as compression rate and number of pixels for recording, a necessary spatial frequency characteristic depends on the setting.
For example, when it is possible to select one million pixels and two million pixels as the number of image-picking up pixels, the necessary AF frequency characteristic for them differs as shown in FIG. 8.
Therefore, a problem of AF operation same as the case in which a camera corresponds to high vision and standard TV occurs as previously described.
Moreover, the performance of a lens depends on the focal length, focus position and aperture stop and they are also factors of change in resolving powers of the lens.
Therefore, frequency characteristics necessary for AF are also changed due to a lens state. For example, a spatial frequency to be resolved differs between WIDE (wide side) and TELE (telephoto side) for the focal length as shown in FIG. 9.
In general, a spatial frequency to be resolved in WIDE is higher than in TELE because an object image becomes more minute in WIDE than in TELE.
Nowadays, while change in zoom to high magnification advances, the difference between spatial frequencies to be resolved at WIDE end and TELE end tends to increase.
Moreover, as shown in FIG. 10, a spatial frequency to be resolved depends on F No. of an aperture stop. Therefore, a phenomenon same as the case previously described occurs in AF.
Furthermore, the resolving power of a lens may be changed depending on a focus position. FIG. 11 is an example in which spatial frequencies to be resolved at focus positions differ.
Therefore, a problem same as the case previously described occurs in AF.
Various problems of AF occur in a lens and an image pickup apparatus because a frequency to be resolved differs in accordance with the type or state of a lens to be set like this and the type or image pickup state of a camera, respectively.